1. Field of the Invention
The invention relates to electrical apparatus, and more particularly, to apparatus for protecting electrical circuits from damage due to ground faults.
2. Description of the Prior Art
In designing circuits for the transmission and distribution of electrical power, it is customary to provide means for protecting the conductors and electrical apparatus being supplied through the conductors from overcurrent conditions. It has gradually been recognized that devices employed for overcurrent protection are not sufficient to prevent damage resulting from ground faults; that is, short circuits between one or more of the electrical conductors and objects connected directly or indirectly to ground. The amount of current which flows through a ground fault may be below the level required to operate the overcurrent protective devices. However, this ground fault current can result in high transient overvoltages throughout the system as well as arcing conditions leading to fire or explosion.
Various methods of detecting and correcting ground faults are currently being used. A current transformer is usually positioned to surround the conductors of the circuit being protected. This transformer, also referred to as a current monitor, produces an output signal upon occurrence of a ground fault condition at a point downstream from the current monitor on the circuit being protected. The monitor is usually connected to a separate device known as a sensor which includes a switching device such as a relay actuated by the output from the current monitor when a ground fault occurs. The contacts of the relay are then used to interrupt the supply of electrical power to the circuit being protected. While it is possible to obtain excellent protection from ground faults through the use of this system, it has sometimes not been included in the design of the electrical distribution systems due to the cost of providing such ground fault protection. The expense of providing and installing two separate units, combined with the complexity of routing the connecting circuitry and supplying power wiring for the relay have often resulted in the elimination of ground fault protection from some circuits. It would therefore be desirable to obtain a simply, compact, ground fault protective device for actuating a circuit interrupter which is low in cost and simple to install.